Hi Boostug,
Did you get details of the offer in writing? If so, check the small print and see if it concures with the information that you have been given over the phone. If you do not have it in writing, your only hope may be an advert that offered this deal to support your claim.
If you do not have any written evidence of the 'deal', you will be unlikely to get your �400, although it is always worth talking to the same underwriter who offered you the �400 and see what he says.
Lastly, the comment about 'offsetting VAT' sounds completely wrong, particularly if you are told that it "will not appear in the figures" and your payments are exactly the same as your credit agreement states.
In your situation, I would write a letter to the Underwriting Manager, setting out your complaint and asking for a lot more detail about how this �400 has been added to your deposit and VAT, without any notification in writing, nor downward adjusting of your monthly payment..
If you write your letter politely enough and do a good job of making the lender / dealer realise they are coming across 'dodgy' and misleading, you just may get your �400 as a 'Shut up' payment (it happens a lot). Just remember, of course, that rudeness and threats (like court) in a letter will probably get you nowhere, in a situation like this.
The best lesson learnt here is that you should get all offers in writing and read the smallprint before you agree. Sadly, the energy to deliver 'offers' soon disappears once the transaction concludes with many firms.
Good luck to you.